This study is being performed to determine the safety and effectiveness of the drug, YM087, in treating patients with congestive heart failure whom also have low blood sodium levels. Patients with low sodium levels may experience symptoms such as muscle cramps, tiredness, fatigue, headaches, loss of appetite, nausea, agitation, feeling of being cold, seizures, and coma. At this point in time there is no reliable way to increase a patient's low sodium level. Current treatment involves very strict fluid restrictions and the use of diuretics (such as Lasix). A low serum sodium level needs to be increased gradually, if done too quickly the nerve endings in the body may be damaged. It is hoped this medication will be useful in increasing the blood sodium level gradually and keeping the level up. There is no placebo involved in this study, everyone enrolled will receive the active medication at one of four different dosages which will be determined by random assignment (by chance, as with the flip of a coin). Persons with congestive heart failure who have low blood sodium levels, between 115 and 132 mEq/L, will be asked to participate in this study. Patients will be admitted to the GCRC for four to six days. The first day is observation with a fluid restriction implemented, blood draws, urine collections, and a physical exam performed. If the serum sodium level is below 132 mEq/L during this day, the next morning a bolus dose of 20 mg of study medication would be given followed by a continuous infusion over 48 hours. If on the second day of the continuous infusion the average serum sodium is above 135 mEq/L the study would be stopped, if not the study would be continued at the choice of the doctor. The patient may be given a second bolus dose of the study medication followed by another 48 hours of continuous infusion at doses that would be determined by the primary doctor. At the end of the second 48 hours of continuous infusion the study would be stopped. The patient would be required to stay in the hospital for 24 hours of observation and would need to return to the out-patient clinic for a follow up exam one week later. Throughout the length of the study blood and urine samples would be gathered frequently so the patients' response to the study medication can be carefully monitored.